ARP comes into play when you, for example, head over to a friend's house, pull out your laptop, and try to use the wireless to surf the web. One of the first things that probably needs to happen is determining the MAC address of the gateway (probably your friend's router), so that the Ethernet packets containing all those IP[TCP[HTTP]] requests you want to send out to the Internet know how to get to their first hop, the gateway.

Your laptop finds out the MAC address of the gateway by asking. It broadcasts an ARP request for "Who has IP address 192.168.1.1", and the gateway broadcasts an ARP response saying "I have 192.168.1.1, and my MAC address is xx:xx:xx:xx:xx:xx". Your laptop, armed with the MAC address of the gateway, can then craft Ethernet packets that will go to the gateway and get routed out to the Internet.

But the gateway didn't really have to prove who it was. It just asserted who it was, and everyone listened. Anyone else can send an ARP response claiming to have IP address 192.168.1.1. And that's the ticket: if you can pretend to be the gateway, you can control all the packets that get routed through the gateway and the content returned to clients.

The gateway router, like most modern routers, is bridging between the wireless and wired domains, so ARP packets get broadcast to both domains.

Step 2: Enable IPv4 forwarding

kid-charlemagne wants to be receiving packets that aren't destined for it (eg the web traffic). Unless IP forwarding is enabled, the networking subsystem is going to ignore packets that aren't destined for us. So step 1 is to enable IP forwarding. All that takes is a non-zero value in /proc/sys/net/ipv4/ip_forward:

root@kid-charlemagne:~# echo 1 > /proc/sys/net/ipv4/ip_forward

Step 3: Set routing rules so packets going through the gateway get routed to you

kid-charlemagne is going to act like a little NAT. For HTTP packets heading out to the Internet, kid-charlemagne is going to rewrite the destination address in the IP packet headers to be its own IP address, so it becomes final destination for the web traffic:

For HTTP packets heading back from kid-charlemagne to the client, it'll rewrite the source address to be that of the original destination out on the Internet.

-t says we're specifying a table. The nat table is where a lookup happens on packets that create new connections. The nat table comes with 3 built-in chains: PREROUTING, OUTPUT, and POSTROUTING. We want to add a rule in the PREROUTING chain, which will alter packets right as they come in, before routing rules have been applied.

What packets

That PREROUTING rule is going to apply to TCP packets destined for port 80 (-p tcp --dport 80), aka HTTP traffic. For packets that match this filter, jump (-j) to the following action:

The rule

If we receive a packet heading for some destination, rewrite the destination in the IP header to be 192.168.1.200 (NETMAP --to 192.168.1.200). Have the nat table keep a mapping between the original destination and rewritten destination. When a packet is returning through us to its source, rewrite the source in the IP header to be the original destination.

In summary: "If you're a TCP packet destined for port 80 (HTTP traffic), actually make my address, 192.168.1.200, the destination, NATting both ways so this is transparent to the source."

One last thing:

The networking subsystem will not allow you to ARP for a random IP address on an interface -- it has to be an IP address actually assigned to that interface, or you'll get a bind error along the lines of "Cannot assign requested address". We can handle this by adding an ip entry on the interface that is going to send packets to pixeleen, the test client. kid-charlemagne is wired, so it'll be eth0.

jesstess@kid-charlemagne:~$ sudo ip addr add 192.168.1.1/24 dev eth0

We can check our work by listing all our interfaces' addresses and noting that we now have two IP addresses for eth0, the original IP address 192.168.1.200, and the gateway address 192.168.1.1.

Step 4: Set yourself up to respond to HTTP requests

kid-charlemagne happens to have Apache set up. You could run any minimalist web server that would, given a request for an arbitrary resource, do something interesting.

Step 5: Test pretending to be the gateway

At this point, kid-charlemagne is ready to pretend to be the gateway. The trouble is convincing pixeleen that the MAC address for the gateway has changed, to that of kid-charlemagne. We can do this by sending a Gratuitous ARP, which is basically a packet that says "I know nobody asked, but I have the MAC address for 192.168.1.1”. Machines that hear that Gratuitous ARP will replace an existing mapping from 192.168.1.1 to a MAC address in their ARP caches with the mapping advertised in that Gratuitous ARP.

We can look at the ARP cache on pixeleen before and after sending the Gratuitous ARP to verify that the Gratuitious ARP is working.

Bam. pixeleen now thinks the MAC address for IP address 192.169.1.1 is 0:30:1b:47:f2:74, which is kid-charlemagne’s address.

If I try to browse the web on pixeleen, I am served the resource matching the rules in kid-charlemagne’s web server.

We can watch this whole exchange in Wireshark:

First, the Gratuitous ARPs generated by kid-charlemagne:

The only traffic getting its headers rewritten so that kid-charlemagne is the destination is HTTP traffic: TCP traffic on port 80. That means all of the non-HTTP traffic associated with viewing a web page still happens as normal. In particular, when kid-charlemagne gets the DNS resolution requests for lycos.com, the test site I visited, it will follow its routing rules and forward them to the real router, which will send them out to the Internet:

The HTTP traffic gets served by kid-charlemagne:

Note that the HTTP request has a source IP of 192.168.1.111, pixeleen, and a destination IP of 209.202.254.14, which dig -x 209.202.254.14 +short tells us is search-core1.bo3.lycos.com. The HTTP response has a source IP of 209.202.254.14 and a destination IP of 192.168.1.111. The fact that kid-charlemagne has rerouted and served the request is totally transparent to the client at the IP layer.

Step 6: Deploy against friends and family

I trust you to get creative with this.

Step 7: Reset everything to the normal state

To get the normal gateway back in control, delete the IP address from the interface on kid-charlemagne and delete the iptables routing rule:

To get the client machines to believe the router is the real gateway, you might have to clear the gateway entry from the ARP cache with arp -d 192.168.1.1, or bring your interfaces down and back up. I can verify that my TiVo corrected itself quickly without any intervention, but I won't make any promises about your networked devices.

In summary

That was a lot of explanatory text, but the steps required to hijack the HTTP traffic on your home subnet can be boiled down to:

substituting the appropriate IP address and interface information and tearing down when you're done.

And that's all there is to it!

This has been tested as working in a few environments, but it might not work in yours. I'd love to hear the details on if this works, works with modifications, or doesn't work (because the devices are being too clever about Gratuitous ARPs, or otherwise) in the comments.

About

Tired of rebooting to update systems? So are we -- which is why we invented Ksplice, technology that lets you update the Linux kernel without rebooting. It's currently available as part of Oracle Linux Premier Support, Fedora, and Ubuntu desktop. This blog is our place to ramble about technical topics that we (and hopefully you) think are interesting.